Brazing of aluminium products such as aluminium heat exchangers are generally performed by means of continuous atmosphere furnaces which are advantageous technologically and economically.
By a mesh belt circulating through a continuous atmosphere furnace, aluminium products are conveyed into the furnace, in which they successively pass through a moisture drying furnace, front chamber, preheating furnace, heating furnace, cooling chamber, and air cooling chamber, whereby the brazing completes. In the heating furnace, a solder material and flux which have been fitted in advance onto parts of the aluminium products to be brazed, are melted in order of flux first and solder next for having them brazed.
In order to braze the products satisfactorily, it is important that the products which are conveyed into the heating furnace, shall be brought to a brazing temperature promptly and evenly all over the products. For this end, that is, in order to heat the products evenly all over the products in such manner, they must be kept under a certain degree of temperature for a considerable period of time. In pracice, the products are heated to a temperature around melting points of the flux and solder in the preheating furnace, in advance of brazing them in the heating furnace.
While it is improtant as mentioned above to shorten the period of time for heating the products in the heating furnace, it is economical too to shorten a period of time for preheating the products in the preheaing furnace beforehand.
However, it is not easy at all to preheat the easily oxidizable aluminium products rapidly without adversely affecting their qualities.
Those fluxes which are used for brazing aluminium products, are fluoric compounds or alkali metal chlorides containing fluorides, and have a melting point of about 550° C. to 500° C. Solders of Al—Si systme which are generally used for brazing of this kind, have a melting point of 500˜630° C.
In other words, a heating range of a heating furnace of the continuous frunace for brazing aluminium products is 500˜630° C., while a preheating range of a preheating furnace thereof is up to 500° C.
It is known that a heating method primarily made by means of convection heat is more effective than heating primarily by radiation, for heating at a comparatively low temperature of less than 500° C. Therefore, it seems reasonable to employ the heating method primarily made by convection, also in the preheating furnace of the aluminium brazing furnace, heating range of which is up to 500° C.
However, surprisingly, in the preheating furnaces for brazing aluminium products, the heating method primarily made by means of convection has not been employed. This is on account of reasons stated below.
On one hand, a neutral gas such as nitrogen and the like is continuously supplied into a preheating furnace as its furnace atmosphere, so that aluminium products themselves and solders and fluxes for brazing them shall not be oxidized in the preheating furnace, temperature of which has been elevated. On the other hand, oxygen is inevitably brought into the furnace, adhering to the products or adhering to a mesh belt which transports the products. When the preheating furnace is heated by the heating method made primarily by radiation, such oxygen is scarcely dispersed in a furnace atmosphere.
However, when the atmosphere in the preheating furnace is agitated and heated by convection currents produced by fans, the oxygen entered into the furnace as a distrabance thereof is also stirred up and circulated in the preheating furnace, resulting in polluting its atmosphere. Especially, fluxes themselves which are to work as reducing and fluidizing agents, are oxidized at considerably large parts thereof, in advance of working in the heating furnace. Such will largely interfere with a succeeding brazing operation, even though the products are heated rapidly in the preheating furnace.
In view of the above, the object of this invention is to provide a brazing method of aluminium products and a furnace therefor, which are characterized in that the aluminium products can be preheated by the heating method which is made primarily by convection currents, rapidly and without interfering with brazing of the aluminium products in the heating furnace.
In other words, the subject of this invention is to make a heating time shorten so that the brazing of aluminium products can be operated more effectively than before, when they are brazed by heating in a continuous furnace comprising preheating and heating furnaces.